Glial cells respond potently to neuronal damage, as well as neurodegenerative disease, by displaying overt changes in morphology, gene expression, migration, and phagocytic activity. Dysfunctional responses contribute to the progression of devastating neurological diseases, such as Alzheimer's disease and Parkinson's disease, and can also promote the onset of some autoimmune disorders. Despite the importance of glia in defending brain health, remarkably little is known about the molecular underpinnings of responses to neuronal damage. A vital long-term goal is to understand how basic glial immune reactions are triggered in the adult brain in response to damaged and dying neurons. The core cellular events (e.g. glial migration to injury sites and phagocytic clearance of neuronal debris) are highly conserved across species and recent work is revealing striking molecular conservation, as well. This proposal uses a well- established adult axotomy assay in Drosophila to investigate the molecular features of glial reactions; the fly offers a tractable genetic system to manipulate gene expression and function with exquisite cellular and temporal precision in vivo. Our preliminary work has identified a novel role for the evolutionarily conserved Insulin/insulin-like growth factor (IGF)-Like Signaling (ILS) pathway in orchestrating glial reactions to axon injury. Based on our findings, we hypothesize that ILS regulates glial immune responses in two fundamental ways: (1) Basal ILS activity in adult glia ensures that glia express key genes (i.e. the Draper receptor and adaptor Ced-6) required to detect and carry out responses to axon damage, and (2) Acute activation of the ILS pathway at injury sites triggers rapid responses in local glia to ensure that damaged neurons are cleared from the CNS. This proposal will employ powerful genetic-molecular tools to investigate how the ILS pathway contributes to axotomy-induced functions in glia. We will: (Aim 1) define the role of Insulin-like Receptor (InR) activity in each step of the glial response to axotomy, including altered gene expression, glial recruitment to injury sites, and glial phagocytic activity; (Aim 2) determine how insulin-like peptides (ilps), the InR ligands, influence basal expression of Draper and Ced-6, as well as each step of the injury response in local glia; and (Aim 3) define the molecular signaling cascades downstream of InR that are coupled to these important glial responses. This work will provide critical mechanistic insight into how damaged neurons communicate with glia to elicit responses and elucidate intrinsic molecular pathways that control these essential glial functions. Our findings will also offer a novel framework for exploring ILS components as therapeutic targets to treat CNS injury, as well as chronic neurodegenerative conditions.